The present invention relates to a method of making a glass such as a high-refractive-index and low-dispersion glass or an index gradient glass applicable to optical elements for cameras, microscopes, endoscopes, electronic image devices, and micromachine eyes.
Rare earth element-containing glasses possess advantages of having high refractive indices and low dispersion, and being lower in toxicity than lead or the like, and so are now used as the greatly valuable glasses required for high-performance optical systems. In particular, Y, La, and Gd-containing glasses are transparent to light in the visible light region, and so are greatly valuable glasses that are used for lenses, etc. employed with white light. In the present disclosure, the term "rare earth element" is understood to refer to scandium or Sc, yttrium or Y, and 15 lanthanum series, viz., lanthanum or La, cerium or Ce, praseodymium or Pr, neodymium or Nd, promethium or Pm, samarium or Sm, europium or Eu, gadolinium or Gd, terbium or Tb, dysprosium or Dy, holmium or Ho, erbium or Er, thulium or Tm, ytterbium or Yb, and lutetium or Lu.
An index gradient type optical element comprises a medium to which a refractive index gradient and, hence, power (refractive power) is imparted, and is now attracted as an optical element indispensable for the next generation of optical system because its ability to make correction for aberrations is so excellent that the number of lenses forming the system can be reduced.
In general, the greater index difference a glass imparts to a lens medium, the more useful the glass is for an index gradient type optical element. Regarding the ability to make correction for aberrations, for instance, SPIE, Vol. 1780, pp. 456-463 (1992) reports that an index gradient type optical element having a low or negative dispersion profile is excellent in terms of the ability to make correction for aberrations, and so is useful for a white light optical system.
Such an index gradient type optical element is made by several glass-making methods, e.g., sol-gel, ion exchange, and molecular stuffing methods. In particular, the sol-gel method has features that make it possible to obtain glass materials with a large aperture, and enable a profile to be imparted to a polyvalent metal oxide so that the resulting index gradient type optical element can have varying properties, and so captures attention.
JP(A)6171974 discloses a method of using La, Y, and Gd components to make an index gradient type glass having a large index difference or a low or negative dispersion profile, wherein, to obtain a silica glass containing much rare earth element, an Al or other component is added hereto.
A typical method of making a glass material having an index gradient by the sol-gel process is disclosed in JP(B) 93027575, which comprises steps of hydrolyzing a solution composed mainly of an alkoxide of silicon to obtain a sol, adding to the sol a solution containing a water-soluble metal salt for imparting an index gradient thereto, thereby obtaining a wet gel, dipping the wet gel in a solution in which the metal salt has a low solubility, thereby precipitating a metal salt crystallite in the wet gel, dipping the wet gel in a solution in which the metal salt has a high solubility, thereby imparting a metal component gradient thereto in a diametrical direction, and drying and firing the wet gel, thereby obtaining a glass material having an end index gradient. JP(A)6171956 discloses glass production by imparting a concentration gradient to a staring element such as La, Y, and Gd using an inorganic or organic salt, and using a metal alkoxide or its derivative as a raw material for Nb, Ta, Ti, and Zr.
A silica glass containing much rare earth element component is stably vitrified by the addition of a component such as Al thereto. This appears to be because that silica and the rare earth element are stabilized, thereby making their coagulation so unlikely to occur that their crystallization can be well prevented.
Raw materials for rare earth element components, for instance, include metal salts, and alkoxides of rare earth elements. The metal alkoxides of rare earth element components, because of having a very low solubility in organic solvents, require much organic solvent and/or acid for their dissolution. In contrast, the metal salts of rare earth element components have a relatively high solubility in solvents, especially water. Consequently, they can yield a solution having a higher rare earth element concentration as compared with the metal alkoxides, and so are suitable for making a sol containing much rare earth element component. Even when the metal salt, e.g., acetate of a rare earth element component is used, however, relatively much water is needed for its dissolution because its solubility in water or an organic solvent is not very high.
On the other hand, raw materials for aluminum include alkoxides represented by Al(OR).sub.3 where R stands for an alkyl group or their derivative, and metal salts represented as by Al(NO.sub.3).sub.3.9H.sub.2 O.
When an alkoxide of aluminum or its derivative is used for the raw material, gel skeletons can be easily made at room temperature upon mixing with a silicon alkoxide, because the aluminum alkoxide has an Al--O bond in its molecular structure from the first time. In the case of a metal salt of aluminum, on the other hand, aluminum cannot form gel skeletons with silica alone because aluminum is present in the form of aluminum ions formed by dissociation. In other words, when the metal salt of aluminum is used for the aluminum material, the resulting gel is relatively fragile and so susceptible to cracking because the number of gel skeletons is smaller than that obtained by the use of the metal alkoxide. In view of the formation of a difficult-to-crack gel, it is thus preferable to use a metal alkoxide of aluminum or its derivative. When the sol-gel process is applied to glass-making, however, a large amount of alcohol or other organic solvent is needed for dispersing the silicon alkoxide, aluminum alkoxide, etc. by a conventional process to obtain a homogeneous sol, because the aluminum alkoxide has a very limited solubility in ethanol, isopropanol, etc.
For the raw material for the rare earth element component it is preferable to use a solution obtained by dissolving its metal salt in water, and for the raw material for aluminum it is desired to use its metal alkoxide or a derivative thereof. In most cases, the metal salt of the rare earth element needs much water because of its low solubility while the metal alkoxide of aluminum or its derivative needs much organic solvent because of their low solubility. Consequently, the concentration of the gel skeleton component contained in the sol becomes very low. In a thermal treatment process in which a wet gel obtained by gelation of such a sol is thermally treated into glass, the gel contracts very largely, and cracks due to an increased stress upon contraction. This in turn makes it impossible to obtain glass or glass yields extremely worse.
For the introduction of much metal salt of the rare earth element, much water is required. Under conditions where much water is present, however, it is difficult to hydrolyze the alkoxide of aluminum or its derivative in a stable manner because their rate of hydrolysis is very high. The solubility of the metal salt of the rare earth element may be increased by elevating the temperature of water or an organic solvent. However, this causes the hydrolysis of the alkoxide of aluminum to be locally promoted, resulting in precipitation and segregation, and it is consequently impossible to make a homogeneous sol and, hence, glass.
For the production of an index gradient glass using precipitation, it is always required to use the metal salt of the rare earth element. In this case, such problems as mentioned above become unavoidably serious.
An object of the present invention is to provide a method of making a glass of excellent quality in a stable manner without causing gel cracking, in which aluminum indispensable for the stabilization of a rare earth element and silica glass is introduced into the silica glass so as to enable the rare earth element that is a substance contributing greatly to the refraction of index of glass to be introduced in a large amount.